1. Field of the Invention
The present invention relates to a control system comprising a controller and field devices distributed in equipment, which are interconnected via a transmission line so as to construct a network.
2. Description of the Related Art
In equipment represented by manufacturing apparatus and inspecting apparatus used in the field of semiconductor production, analyzing apparatus and automation apparatus used in the field of medical service, etc., it has been conventional that an equipment controller board for controlling the entire equipment and I/O boards are both arranged in a single rack, and an equipment controller controls input means, such as sensors, and output means, such as motors and valves, in a centralized manner. Also, the input/output means, such as motors, valves and sensors, in the equipment are connected to the I/O boards in the rack in one-to-one relation by wiring of special cables. The equipment controller then controls the input/output means via the I/O boards individually. In such a conventional arrangement, therefore, the input/output means cannot be controlled without the equipment controller.
Recently, field devices incorporating microprocessors therein have been developed and put into practice with an improvement of the semiconductor integrated circuit technology. Input/output means, such as motors, valves and sensors, are connected to the field devices, which are made intelligent to be able to control the input/output means by the incorporated microprocessors. Thus, the field devices take over a portion of the measuring and control functions that have so far been performed by an equipment controller. According to one proposal, a transmission line interconnects an equipment controller and a plurality of field devices such that these field devices are connected to the same transmission line in multi-drop fashion. The equipment controller and the field devices construct a system in which two-way communication is carried out via a network using digital signals, whereby the amount of wiring required in equipment is cut down.
In a field network, generally, each field device performs its own operation-corresponding to a position at which the field device is connected to the network, and even the field devices having the same function perform different operations from each other depending on their connected positions. By way of example, in a transporting apparatus comprising a first belt conveyor driven by a first motor and a second belt conveyor driven by a second motor, the transport distance of the first belt conveyor is 10 cm and the transport distance of the second belt conveyor is 20 cm. The first motor is controlled by a first field device, and the second motor is controlled by a second field device. Also conventionally, to uniquely define the first and second field devices connected to the transmission line, a specific ID is assigned to each field device.
In a conventional control system using field devices, however, the following problem arises when a failure or any other trouble occurs. For checking the operation of the field devices and the input/output means including a failed one, it is often tried in such a case to attach another normally operating field device to the system instead of the existing one. Because of the field devices having different IDs, however, the attached field device must be set again to the ID corresponding to the connected position, and hence maintainability is poor when the work of checking the operation is performed upon the occurrence of a failure. A similar problem also arises in assembly. When only one field device is usable and employed to check the operation of the field device and the input/output means by attaching it to the system at different positions in turn, the attached field device must be set again to the ID corresponding to the connected position, and therefore maintainability is also poor in the work of checking the operation during assembly.